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AmpCasq: an OPA1622 integrated headamp project
AmpCasq: an OPA1622 integrated headamp project
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Old 4th March 2017, 04:57 PM   #1
AIM65 is offline AIM65  France
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AmpCasq: an OPA1622 integrated headamp project
Default AmpCasq: an OPA1622 integrated headamp project

Hi All,
Like some of you I followed with great interest the thread 'New Audio Op Amp - OPA1622' by TIís Johnnc124. I wasnít initially specifically interested by headamps but as the OPA1622 was simple to implement and was supposed to provide good results I decided to setup a project with it. More than building an headamp this project was aimed to confirm I could achieve designing a PCB with a CAD software and also soldering SMD devices, including ones in DFN or QFN package.

Iíhave finally been through all steps, Iím happy with the result (OPA1622 sounds very good !) and Iím now confident to move to projects that will require complicated PCB and good soldering capabilities.
Now, I have to finish assembly and testing of a Salas DGC3 in order to do some comparisons!

The main requirements for AmpCasq where :
  • It should stand in a small aluminum box, in order to sit on desk
  • It should be powered by an AC adapter I had, which provided 17VDC
  • It should have a volume control
  • It should have RCA and jack 3.5 inputs
  • It should have jack 3.5 & 6.35 outputs
  • It should allow me to experiment crossfeed
  • Final price should stay under control

About the schematics:
Regulators:
Ampcasq was designed in September 2016, at this time LM27762 was announced but not available. So, I did stay with LM27761 for -5V and LP38798 for +5V; LP5907 was a bit short on current. The 7805 lifted by R17/R12 was added to limit LM27761 input voltage to 5.5V, its max value. 7805 allow Ampcasq to be powered by any DC source from 10V to 35V, 300mA max.
Power Input:
D1 to protect against bad connection.
D2, R23, R10, R11, C24 to manage the Enable pin of the OPA. Objective was to avoid noise in the can at power up/down. This partly works: mute the output when enable Low (<~0.8V) but canít avoid noise in can when power supply capacitors are discharching and OPA1622's supply getting out of tolerance. On power off with SW1, OPA1622 is muted as C24 is quickly discharged through R23, D2 and SW1. D2 is a Schottky, not for speed but for low drop (Vf close to 0.3V).
Crossfeed:
It is based on an Xen-Audio (EUVL) paper: Xen Audio cross feed.
Thereíre three positions for the switch: Low Xfeed Ė No Xfeed Ė High Xfeed. Low Xfeed is Ok, High is exaggerate: thereís too much bass boost. After some listening sessions I finally do not use the Xfeed.
Gain + out
Not much to say.


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Old 4th March 2017, 05:11 PM   #2
AIM65 is offline AIM65  France
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AmpCasq: an OPA1622 integrated headamp project
Default Some pictures

It's mostly finished, but a (very) important thing is missing : a nice volume button
Attached Images
File Type: jpg AmpCasqV3_PCBF_S.jpg (168.3 KB, 1388 views)
File Type: jpg AmpCasqV3_PCB_B_S.jpg (142.4 KB, 1326 views)
File Type: jpg AmpCasqV3_Board_S.jpg (161.6 KB, 1332 views)
File Type: jpg AmpCasqV3_Pluged_S.jpg (148.7 KB, 1305 views)
File Type: jpg AmpCasqV3_Box_Back_Open_S.jpg (121.9 KB, 488 views)
File Type: jpg AmpCasqV3_Box_Back_S.jpg (120.1 KB, 321 views)
File Type: jpg AmpCasqV3_Box_Front_S.jpg (127.8 KB, 254 views)
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Old 4th March 2017, 05:33 PM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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I dropped a huge clanger when I made a similar circuit.
I hadn't kept the power supply ground separate from the audio ground and got massive hum. I had randomly mixed in power supply ground with audio ground.
The power supply needs to be on its own on one corner/side of the pcb.
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Old 4th March 2017, 05:58 PM   #4
AIM65 is offline AIM65  France
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AmpCasq: an OPA1622 integrated headamp project
Right, I had hum issues with the first version of pcb, then I spent some time on the ground plane and now it's dead silent.

Lm27761 is very sensitive to ground, I've spend a lot of time on it, now it as it's own ground plane on component side and it behave very well up to 200mA.
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Old 4th March 2017, 06:44 PM   #5
AIM65 is offline AIM65  France
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AmpCasq: an OPA1622 integrated headamp project
Default Pcb

I've some spare pcb available for free, only shipping costss will be charged.
Updated BOM will be ready soon.
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Old 4th March 2017, 09:30 PM   #6
Shaq888 is offline Shaq888  Ukraine
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If you want to get a 100% from 1622 you'll need to optimize the layout. The heavier load you want to drive - there is more demand to placing and choosing supply decoupling caps.
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Old 5th March 2017, 01:12 PM   #7
AIM65 is offline AIM65  France
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AmpCasq: an OPA1622 integrated headamp project
Thanks for comments Sergey888.
To be honest I wasn't planning to do a high end headamp, only to go through all steps of project in order to have a finished and usable headmap, and also something I could share on DIYA.
Decoupling caps of OPA supply has been choosed to provide good value for price : nothing exceptional but Pana FR for ecap and Murata for the 100n MLCC Y7R 50V, 805 form factor. At 5V bias this cap is at 100% of its nominal value. Murata does publish the full specs of its capacitors on its website this is very useful. Theyíre located as close the chip as I could imagine being able to weld themÖ
Iív done some measurements with a scope with various load, theyíre consistent with OPA1622 datasheet. It drive 32R paralled with 1nF up to 3.1Vp, while staying stable and it operating a cool temperature. Heatsink made in PCB is enough.

Youíre right, so does nigelwright7557, layout is perfectible, Iíll keep that in mind for the next project !
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Old 5th March 2017, 01:59 PM   #8
johnc124 is offline johnc124  United States
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Great project! Very happy to see people using my little chip. OPA1642 is also an ideal choice for buffering the volume control pot.
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Old 5th March 2017, 10:13 PM   #9
adydula is offline adydula  United States
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Aim65,

PM sent and paypal for a board....thanks! Looking forward to building up this amp.

Thanks
Alex
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Old 5th March 2017, 10:50 PM   #10
Shaq888 is offline Shaq888  Ukraine
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Quote:
Originally Posted by AIM65 View Post
To be honest I wasn't planning to do a high end headamp, only to go through all steps of project in order to have a finished and usable headmap, and also something I could share on DIYA.
Decoupling caps of OPA supply has been choosed to provide good value for price : nothing exceptional but Pana FR for ecap and Murata for the 100n MLCC Y7R 50V, 805 form factor. At 5V bias this cap is at 100% of its nominal value. Murata does publish the full specs of its capacitors on its website this is very useful. Theyíre located as close the chip as I could imagine being able to weld themÖ
Iív done some measurements with a scope with various load, theyíre consistent with OPA1622 datasheet. It drive 32R paralled with 1nF up to 3.1Vp, while staying stable and it operating a cool temperature. Heatsink made in PCB is enough.

Youíre right, so does nigelwright7557, layout is perfectible, Iíll keep that in mind for the next project !
Improving it does not mean that you need to spent tons of $$ on part - you just need to add and properly place a few fairly cheap components.
First of all 100nF caps have rather high impedance to really do a lot. It is necessary to provide a local low impedance path for at least high-ish audio frequencies. Something low-ESR around 220-470uF would be capable of doing it. And the reason for that not that much of a creeping signal products through a finite PSRR (also this is also can be a limiting factor) but inductive coupling supply rails, that caring highly distorted current, to signal lines. The longer those lines are - the more likely you'll get a trouble because of it. It is not the greatest idea to let them flow to the power supply.
The difference i'm talking about is not something you'll be able to see with a scope.
Have a look on some of my measurement results. On this level i managed to achieve something like 3-4dB improvement just by moving caps closer and keeping signals line further from supply.
Universal buffer/headamp based on OPA1622
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